1. Field of the Invention
This invention relates to a process for preparing a ceramic film, and more particularly to a process for preparing a ceramic film which enables rapid film formation. In this process pore size can be controlled and the resultant film can be both uniform and thick.
2. Prior Arts
Ceramic films are used as thermal insulating materials, filtering medium, or the like, and recently developed porous glass is expected to be widely used as a high-temperature gas separating material or a sinterable matrix for a sintered material of high density.
As a method for preparing ceramic powders, there can be mentioned a vapor phase method, a liquid phase method or a solid phase method. Of these methods, the vapor phase method is the most desirable in that the purification of the raw material can be effected easily so that ceramics of high purity can be obtained. Also, surface contamination of fine particles with large specific areas rarely occurs in this method.
In the usual method for preparing a film of ceramics, material powders are synthesized by a liquid and a solid phase reaction, and the resultant solid is then pulverized, sieved and sintered. This method, however, is not suitable for preparation of a ceramic film of high purity and of fine pores.
Recently, a method of sintering ultrafine particles synthesized by a vapor phase reaction was proposed (Ceramics, Vol. 19, No. 6 (1984), p. 469). The production process of this method, however, is very complicated and there are possibilities of impurity contamination. Furthermore, in the case of a porous glass having ultrafine pores of 100A or less, it may be possible to prepare a material of SiO.sub.2 having ultrafine pores by eluting sodium borate from boron silicate glass of entangled structure, but this method is not general and can not be widely employed.
Thus, there has not heretofore been a general method which is applicable to a wide range of films having a high purity and desired porosity. In connection to this, it is to be noted that, as a method for preparing a sinterable matrix to obtain a dense sintered material, there has been known a method for preparing the porous glass as described above from silica, but again this method is not general.
As a film forming technique for directly obtaining a ceramic film, there is the CVD (Chemical Vapor Deposition) method or PVD (Physical Vapor Deposition) method. In the case of ceramics, the PVD method is such that the sputtering process using an oxide as a target, is employable, but this is disadvantageous in that the film formation speed is as low as 1 .mu.m/hr or less. Whereas, CVD method has an advantage that the film formation speed is high and the obtained film has a desired crystallizability so that multi-component film can easily be prepared. Thus, this method is the most desirable among the conventional methods.
The CVD method, however, has another drawback. According to this method, although the film formation speed can generally be increased by increasing the concentration of the reactive material the uniformity of the film is deteriorated as the concentration is increased and particles which suppress the formation of the film are undesiredly produced in the vapor phase when the concentration exceeds a certain limit. Thus, in relation with such a possible undesired result of production of particles in the vapor phase, the conventional CVD method, in which solids are allowed to grow only on the substrate, has an upper limit in the concentration of the reactive materials. By this reason, the film formation speed is suppressed to 20 .mu.m/hr or less and if uniform film formation is desired, it is further suppressed to approximately 1 to 5 .mu.m/hr.
The problems involved in the conventional techniques to be obviated by the present invention are summarized as follows:
(1) The conventional methods include a number of steps and the process is complicated so that there is a possibility of contamination. In other words, there is no method which is simple and energy efficient.
(2) The conventional process which is capable of directly obtaining a dense ceramic film is disadvatageous because the film formation speed is low.